If we make a diagram which compares the brightness and colour of stars, a very strange thing happens.
If we represent each star by a point having a particular brightness and colour, we find that the points gather together only in certain parts of the plot.
This type of diagram is called the Hertzsprung-Russell (or HR) diagram after the two astronomers who discovered this effect.
This is how the diagram looks for a sample of stars near the Sun.
The position of a star in the diagram will change as its brightness and colour change with time. We don’t have time to watch individual stars change position, but by plotting the position of thousands of stars we get a good idea of which combinations of brightness and colour stars are allowed to have during their life-cycle.